Organisms selectively filter sensory input, increasing their ability to detect some stimuli at the cost of decreasing their ability to detect others. This process, often referred to as "selective attention", is crucial to normal functioning in many contexts, and deficits in selective attention play a significant role in a number of pathological conditions, including attention-deficit hyperactivity disorder, unilateral visual neglect, and Alzheimer's disease. The research proposed here takes advantage of a powerful model system that uses operant procedures to test visual predators searching for naturalistic, hard-to-find prey under well-controlled conditions. This paradigm will be used to study the role of selective attention in determining responses to complex visual stimuli. This is of particular concern because the simpler geometrical stimuli do not place a sufficient demand on information processing capacity to enable selective attention effects. [unreadable] [unreadable] Emphasis will be placed on developing a more complete understanding of the regulatory dynamics of attentional processes and the relationship between different modes of eliciting attention. Key questions will include: What factors determine the stimuli to which an organism attends at any given moment? What are the cues that elicit the occurrence of an attentional state? Is attention to a particular stimulus configuration a single, unified phenomenon that is potentially elicited by a number of different cues, or can attention be primed by cues that engage different cognitive mechanisms? By addressing such issues in a naturalistic context with suitably complex stimuli, the proposed approach has unique potential for developing important insights into the nature and mechanisms of selective attention, making a significant contribution to neglected aspects of attention and visual cognition. [unreadable] [unreadable]